1. Field of the Invention
The invention relates to an electrographic stylus writing apparatus.
In an electrographic stylus writing apparatus it is usual to have a plurality of exposed styli and to move a recording medium past the exposed styli. The record medium is conventionally a conductive material having a dielectrically coated surface which is presented to the styli as the record medium moves past the head. The head is responsive to input signals and arranged to select individual styli or groups of styli so as to form a surface charge image on the recording medium, which image is later developed elsewhere in the printing system. Considerable electrical energy is often required to ensure that the relative potential between styli and record medium is achieved to enable writing by causing ionisation of the gap between the styli and the medium so that the surface charge image can be formed. Considerable energy is lost in conventional systems in producing the relative potentials due to inefficiencies in the driving systems. The actual energy involved in charging the air gap at the stylus tip can be very little.
2. Description of the Prior Art
In U.S. Pat. No. 3,859,960 for example, selected styli and a backing plate behind the record medium are energised simultaneously with signals of opposite polarity to enable writing. In U.S. 3,611,419, the front plates and styli are energised by signals of opposite polarity, on the styli and front plates, and a capacitive coupling is formed with the record medium to enable writing. In both U.S. Pat. Nos. 3,859,960 and U.S. 3,611,419, it is normally required in practice, to avoid false writing, to energise groups of styli separated by groups of non-energised styli. This is achieved as specifically described in U.S. Pat. No. 3,611,419 by making sure adjacent front plates are not energised simultaneously.
In U.S. Pat. No. 4,030,107, an R-C matric arrangement is disclosed for energising an array of gas discharge electrodes..